The invention relates generally to the detection of shorts in various electrical circuits and more particularly to a method and apparatus for detecting shorts with a lower risk of damaging the circuits during a detection procedure.
It is often desirable to test power conversion circuits for integrity. Generally, a "power conversion circuit" refers to any circuit that renders a change in electrical power, such as a step-up transformer, a step-down transformer, an inverter, a rectifier, a switching bridge, or the like. Power conversion circuits find applications in many devices, such as AC motor drives for example. Typically in an AC motor drive, three phase AC power is converted to DC power and conversion circuits, such as bridges, are coupled between positive and negative DC buses. The conversion circuits are operated in a predetermined manner to generate a reasonable facsimile of AC power having the desired voltage and frequency using various techniques, such as pulse width modulation. Many power conversion circuits comprise complex semiconductor circuits that must be manufactured under very precise conditions and are subject to defects or failure due to overloading.
Of particular importance in many power conversion circuits is avoidance of shorts. Of course, shorts may cause the power conversion device to malfunction. Further, shorts in power conversion devices can cause damage to other components coupled to the power conversion device, such as motors, and may even cause physical harm to nearby personnel due to electrical shock or explosion. Accordingly, power conversion circuits and other electrical circuits are often subjected to short circuit test procedures after assembly.
It is known to accomplish short circuit testing of power conversion circuits by charging the DC bus and subsequently firing the gates of the circuit elements of interest in sequence while observing for symptoms of a short. However, this procedure has disadvantages. First, the timing of firing the various gates must be executed precisely or the results of the test procedure will not be accurate. It is difficult in a practical sense to achieve the required timing precision. Second, if the DC bus is charged beyond a relatively low level when the gates are fired, the bus capacitors will discharge at a very high current level, often in excess of the ratings of the circuit elements such as integrated gate commutated thyristors (IGCTs). Of course, such currents can damage the circuit elements. For these reasons, the short circuit test procedure described above is difficult to apply and often destructive.